is this a problem for fast computers?

No, because the virus diagnostics will also run faster.

Also a lot of malware anymore is designed so that it only uses idle resources or a fraction or resources. You don't really notice a slowdown until multiple programs built up, unless you get a badly designed one.

It's not a problem if you know your computer is protected. In my case, that mainly means using Firefox with certain addons, turning on Windows firewall and being behind a modem AND a router. If I were a frequent visitor to Facebook, I'd use anti-virus and an active anti-spyware program. However, I rarely go anywhere that could cause my computer harm and if I am suspicious of a link I want to follow, then I use a sandbox so I know that emptying it will get rid of any malware.

My children do go to Facebook fairly often and also use their laptops on a school network so they run more security software than I do and wisely so. The only problem any of us have had is one of them thought a friend had sent a link and told a security program that it was okay to unblock it and then had to run Malwarebytes to get rid of the infection and change a password - no major drama.

Unless it's a virus designed to destroy your computer or deliberately slow it down, I don't think how good your computer is, is really very relevant any more. More of the malware around today is designed to be unobtrusive so it can steal personal information or enslave your computer to be used as a zombie in an attack on other computers. Bringing your computer to a standstill or deleting data brings it to your attention and raises the chance that the culprit will be discovered quickly.

Much more important than the quality of your computer is the quality of your protection of it and your sensible surfing.

I've never though about it but you make a good point. The cpu speed and such isn't going to make it "hurt" your system any faster but If you do have a high end computer I soppose you would have a hard time noticing your computer slowing down.